Klaips et al. outline the pathways and molecular mechanisms of cellular protein homeostasis, or proteostasis, and discuss how a decline in proteostasis during aging contributes to disease.

According to the prion hypothesis, atypical phenotypes arise when a prion protein adopts an alternative conformation and persist when that form assembles into self-replicating aggregates. Amyloid formation in vitro provides a model for this protein-misfolding pathway, but the mechanism by which this process interacts with the cellular environment to produce transmissible phenotypes is poorly understood. Using the yeast prion Sup35/[PSI + ], we found that protein conformation determined the size distribution of aggregates through its interactions with a molecular chaperone. Shifts in this range created variations in aggregate abundance among cells due to a size threshold for transmission, and this heterogeneity, along with aggregate growth and fragmentation, induced age-dependent fluctuations in phenotype. Thus, prion conformations may specify phenotypes as population averages in a dynamic system. Prion proteins adopt a spectrum of conformations or strains, which create phenotypes of distinct severity and stability in vivo (1-3). These phenotypes are linked to the assembly of the protein into aggregates that, at unique rates, template the conversion of newly-made prion protein to a similar state and are fragmented (4). But, how do these biochemical events translate into distinct phenotypes? One possibility is an "abundance-based" model, in which phenotypes are linked to an equilibrium between aggregated and soluble prion protein that determines protein activity and the number of heritable prions (propagons) (5,6). However, the conversion and fragmentation reactions also create heterogeneity in aggregate size, raising the possibility of a second, "size-based" model in which a subpopulation of aggregates establishes and propagates phenotypes (7).To distinguish between these models, we focused on the [PSI + ] Weak and [PSI + ] Strong conformations of the yeast prion protein Sup35, which create phenotypes of different stabilities in vivo (8). To sustain these phenotypes in a dividing culture, Sup35 protein in the prion conformation must be inherited (7). To test whether conformational differences impact phenotypic stability by altering protein transmissibility, we monitored Sup35-GFP transfer to daughter cells. Using fluorescence loss in photobleaching (FLIP), a [PSI + ] Weak strain transferred half as much Sup35-GFP (~15% vs. ~30%; Fig. 1A) and contained ~50% fewer ** This manuscript has been accepted for publication in Science. This version has not undergone final editing. Please refer to the complete version of record at http://www.sciencemag.org/. The manuscript may not be reproduced or used in any manner that does not fall within the fair use provisions of the

The proteostasis network has evolved to support protein folding under normal conditions and to expand this capacity in response to proteotoxic stresses. Nevertheless, many pathogenic states are associated with protein misfolding, revealing in vivo limitations on quality control mechanisms. One contributor to these limitations is the physical characteristics of misfolded proteins, as exemplified by amyloids, which are largely resistant to clearance. However, other limitations imposed by the cellular environment are poorly understood. To identify cell-based restrictions on proteostasis capacity, we determined the mechanism by which thermal stress cures the [PSI+]/Sup35 prion. Remarkably, Sup35 amyloid is disassembled at elevated temperatures by the molecular chaperone Hsp104. This process requires Hsp104 engagement with heat-induced non-prion aggregates in late cell-cycle stage cells, which promotes its asymmetric retention and thereby effective activity. Thus, cell division imposes a potent limitation on proteostasis capacity that can be bypassed by the spatial engagement of a quality control factor.DOI: http://dx.doi.org/10.7554/eLife.04288.001